The present invention relates to an eddy current testing device with an electronic balancing loop for use in non-destructive testing.
Non-destructive testing by eddy currents involves producing a variable magnetic field with the aid of a primary winding, subjecting a member to be tested to this magnetic field, sampling a measuring voltage at the terminals of a secondary winding positioned in the vicinity of the tested member and analyzing this voltage. The primary and secondary windings may coincide or may form a bridge. Any defects in the member (change of size, variations in the electrical conductivity, variation in the magnetic permeability, cracks, etc) modifies the phase and intensity of the eddy current circulating in the member and correlatively changes the measuring voltage.
In most cases, the measurement is of a differential nature. For this purpose, a satisfactory standard member is exposed to an identical magnetic field to that to which the member to be tested is exposed and the analyzed voltage is formed by the difference between the voltage corresponding to the tested member and the reference voltage corresponding to the standard. The means making it possible to produce this reference voltage and form the differential voltage constitute the balancing means.
An eddy current non-destructive testing device with balancing means is in the form illustrated in FIG. 1. A sinusoidal signal generator 10 is followed by an amplifier 12, which supplies a probe 14 in the vicinity of which pass the members 16 to be tested. A second probe 14', identical to the first and also excited by amplifier 12, is positioned relative to a standard member 16' in the same way as probe 14 is positioned relative to the members to be tested 16. The voltage sampled at the terminals of probe 14 forms the measuring voltage and that sampled at the terminals of probe 14' the reference voltage.
These voltages are amplified by amplifiers 18, 18', which supply voltages Vm and Vr, which are applied to the two inputs 20/1 and 20/2 of a differential amplifier 20. The latter is followed by a variable gain amplifier 22, connected to signal processing means 24. These means are able to determine the parts of the differential voltage respectively in phase and in phase quadrature with the exciting voltage supplied by the generator 10. However, it can also be a more complex circuit making it possible to determine the Fourier components of the signal. A circuit 26 makes it possible to process the results (storing, recording the results, statistics, etc).
It would appear from FIG. 1 that the probes are constituted by a single winding. However, in practice, they can have several windings (e.g. arranged in bridge-like manner), but the simplification made here has no effect on the understanding of the invention.
With regards to this type of equipment, reference can be made to U.S. Pat. No. 3,229,198 of H. L. LIBBY, granted on Jan. 11th 1966 and entitled "Eddy current nondestructive testing device for measuring multiple parameter variable of a metal sample".
In certain devices of this type, the processing means 24 are of a digital nature. In this case and as illustrated in FIG. 1, they comprise an analog--digital converter 30, followed by a random access memory 32, addressed by a counter 36, connected to a clock 38. The memory is followed by a digital processing circuit 34. If it is a question of calculating the harmonic components of a signal, this circuit performs a discrete Fourier transformation.
The balancing system shown in FIG. 1 has numerous disadvantages:
it reduces by half the power available for the members to be tested; PA1 it requires two probes, PA1 it makes it necessary for the standard to be permanently exposed to the magnetic field, which in the long term leads to a modification of the reference signal (heating of the material, variation of its magnetic properties, etc).